1. Field of the Invention
The present disclosure relates to an organic light-emitting diode. More particularly, the present disclosure relates to a high-efficiency organic light-emitting diode in which an electron-density-controlling layer is disposed between a light-emitting layer and an electron injection layer.
2. Description of the Related Art
Organic light-emitting diodes, based on self-luminescence, exhibit the advantages of having a wide viewing angle, excellent contrast, fast response time, high brightness, and excellent driving voltage and response rate characteristics, and of realizing a polychromic display.
A typical organic light-emitting diode includes an anode and a cathode, with an organic emissive layer disposed therebetween.
As to the general structure of the organic light-emitting diode, a hole transport layer, a light-emitting layer, an electron transport layer, and a cathode are formed in that order on an anode. Here, all of the hole transport layer, the light-emitting layer, and the electron transport layer are organic films comprising organic compounds.
An organic light-emitting diode having such a structure operates as follows: when a voltage is applied between the anode and the cathode, the anode injects holes which are then transferred to the light-emitting layer via the hole transport layer while electrons injected from the cathode move to the light-emitting layer via the electron transport layer. In the luminescent zone, the carriers such as holes and electrons recombine to produce an exciton. When the exciton returns to the ground state from the excited state, the molecule of the light-emitting layer emits light.
Materials used as the organic layers in organic light-emitting diodes may be divided into luminescent materials and charge carrier materials, for example, a hole injection material, a hole transport material, an electron injection material, and an electron transport material. The light-emitting mechanisms allow the luminescent materials to be classified as fluorescent and phosphorescent materials, which use excitons in singlet and triplet states, respectively.
Meanwhile, when a single material is employed as the luminescent material, intermolecular actions cause the maximum luminescence wavelength to shift toward a longer wavelength, resulting in a reduction in color purity and light emission efficiency. In this regard, a host-dopant system may be used as a luminescent material so as to increase the color purity and the light emission efficiency through energy transfer. This is based on the principle whereby, when a dopant has a smaller energy band gap than a host, which consist of the light-emitting layer, the addition of a small amount of the dopant to the host generates excitons from the light-emitting layer so that the excitons are transported to the dopant, emitting light at high efficiency. Here, light of desired wavelengths can be obtained depending on the kind of the dopant because the wavelength of the host moves to the wavelength range of the dopant.
With regard to the efficiency of organic light-emitting diodes, statistically, there is a 25% probability of forming a singlet state and a 75% probability of forming a triplet state. It would thus be expected that in fluorescent OLEDs only the formation of singlet excitons results in the emission of useful radiation, placing a theoretical limit on the internal quantum efficiency of 25%.
To avoid the disadvantage, Korean Patent Unexamined Application Publication No. 10-2012-0092555 (Aug. 21, 2012) proposes the effective occurrence of a TTF phenomenon, in which singlet excitons are generated through the collision and fusion of two triplet excitons. For this, as shown in FIG. 1, this reference discloses an electroluminescence device in which a blocking layer is interposed between a light-emitting layer and an electron injection layer, with an affinity difference between the electron injection layer and the blocking layer. In this regard, the blocking layer is set to have a triplet energy larger than that of the host of the light-emitting layer so as to confine triplet excitons within the light-emitting layer, whereby the effective occurrence of the TTF phenomenon is induced. In addition, the electroluminescence device employs a material in which respective affinities of both the electron injection layer and the blocking layer satisfy a specific condition.
As described above, the reference document is designed to cause the effective occurrence of a TTF phenomenon in order to provide high emission efficiency for an organic electroluminescence device. To this end, the blocking layer should include a material that is higher in triplet energy than the host, and an aromatic heterocyclic compound of a specific fused ring should be employed in the blocking layer.
In spite of various efforts made to fabricate organic light-emitting diodes having effective luminescence characteristics, however, there is still a continued need to develop organic light-emitting diodes having a higher effective luminescence efficiency by improving the emitting layer.